Entrainment Zone Characteristics and Entrainment Rates in Cloud-Topped Boundary Layers from DYCOMS-II

摘要

The major objective of this thesis is to understand entrainment zone properties and the cloud-top entrainment rates using in situ aircraft measurements. The entrainment zone is defined objectively using a new method based on turbulence perturbations from high-rate turbulence samplings taken during the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study. The Entrainment Interfacial Layer (EIL) of the stratocumulus-topped boundary layer (STBL) is defined as the region near the cloud top where mixing occurs between dry free-troposphere air and moist turbulent air. Although the concept of the entrainment zone is clear, defining the top and bottom altitudes of the EIL from vertical profiles of tracer variables is complicated by many factors including an insufficient number of sounding profiles to provide good statistics. This issue is further complicated by the presence of multiple interfaces near the cloud top often used as a substitute for the EIL. As a result, the region that comprises the entrainment zone is not clearly defined. This study examines the characteristics of several important interfaces such as the cloud top, the inversion layer, the interface between turbulent and non-turbulent layers, and the EIL. The relative heights and depths of these interfaces are studied using a large number of sounding profiles from five DYCOMS-II flights. The characteristics of the EIL jump conditions were also analyzed; their variability illustrates the complexity of the jumps and therefore the uncertainty in entrainment rate calculations. Entrainment rates were estimated from jump conditions obtained in this thesis and compared to previous studies.